In a conventional retractable landing gear assembly the gear retracts into the wheel well after takeoff or extends to a down and locked position prior to landing. The extension and retraction force is provided by a double acting hydraulic actuator connected between the movable trunion and fixed aircraft structure. Once the landing gear is in a retracted position, a mechanical up-lock hook is automatically engaged and hydraulic power is subsequently removed from the hydraulic actuator by a position sensing and control system. In order to extend the landing gear, hydraulic power is sequenced to first disengage the mechanical up-lock device. When the up-lock device is fully disengaged, hydraulic power is then directed to the double acting hydraulic actuator to extend the landing gear to a down and locked position. Once the landing gear achieves a down and locked position, a position sensing and control system removes hydraulic power from the double acting hydraulic actuator.
In the event of a failure in the normal gear extension system, a redundant means of extending the landing gear must be provided. This may consist of a purely mechanical system consisting of cables, pulleys, and brackets that connect the mechanical up-lock hook to a manual handle in the cockpit. Significant manual input force may be required to manually release the landing gear up-lock. This activity increases the flight crew's workload during critical phases of flight.
A disadvantage inherent in existing mechanical landing gear up-lock systems is that the various linkages and components of a mechanical system must be installed and properly adjusted such that the desired release of the landing gear prior to landing gear extension actually occurs. In some cases, because of improper adjustment or installation, the above described up-lock hook has failed to disengage rendering it impossible to extend landing gear for a landing. Such a landing gear extension failure is an extremely disruptive event that results in significant danger to passengers and crew and at best, significant damage to the aircraft as a gear-up landing is executed. What is needed is a simpler landing gear up-lock system that also reduces flight crew workload in the event that the normal extension system has failed. A simpler landing gear up-lock system would also reduce the time required to install the system and reduce the maintenance activity required to inspect and maintain the mechanical backup system.